guys, so, I'm talking to this guy who's offering me a beercan for 150 bucks. He said there's a couple of scuffs here and there. I looked and it isn't anything major. However, one thing's been bothering me: this dirt on the mount-side of the lens. I don't know if it's cleanable or not, but I told him to clean it up and send me a pic afterwards. Check it out

http://farm3.static.flickr.com/2060/2625899227_e608ee96cd_b.jpg

I was just curious as to what I should be looking for when buying used equipment. I'm interested in the 50mm 1.7.
what should I do, look for, and any other tips when attempting to purchase a used equipment. all opinions and thoughts are welcomed. thanks in advance.